In motor vehicles, such as cars or the like, that have between an engine and an automatic transmission mechanism a torque converter, there is a type where the torque converter is equipped with a lock-up-clutch for reducing deterioration of fuel consumption which would be caused by a slippage of the torque converter. The lock-up-clutch has, as operation conditions, a lock-up condition wherein input and output elements are directly engaged, a converter condition wherein the input and output elements are fully disengaged to effect a torque transmission through a fluid, and a slip condition wherein the lock-up-clutch is made in a half-engagement condition to keep a given slip condition.
For controlling the lock-up-clutch, the above-mentioned three operation conditions are suitably switched in accordance with an operation condition of the vehicle, and the switching of the operation modes is carried out by varying a lock-up differential pressure [viz., differential pressure (ΔP (=Pa−Pr) between oil pressure Pa in an apply chamber and oil pressure Pr in a release chamber of the lock-up-clutch, in the following, the differential pressure will be referred to as a lock-up-clutch engaging pressure]. When the lock-up differential pressure ΔP is increased, the lock-up condition takes place, while, when the lock-up differential pressure ΔP is decreased, the converter condition takes place. When the lock-up differential pressure ΔP is between the above-mentioned increased value and the above-mentioned decreased value, the slip condition takes place.
In case of switching the operation condition from the converter condition to the lock-up condition, the pressure increasing is carried out under an open loop control until the pressure reaches the lock-up differential pressure, and thereafter, a slip control under a feed-back control is carried out to induce the lock-up condition. With this, the operation condition can be smoothly shifted to the lock-up condition. In the open loop control of this case, by periodically increasing a predetermined amount of change for increasing the engaging capacity of the lock-up-clutch, the lock-up-clutch engaging pressure is increased as the time elapses.
However, in case of using the control in which the predetermined amount of change is periodically increased to increase the fastening engaging capacity thereby to increase the lock-up-clutch engaging pressure, it is impossible to treat with a case wherein the engine torque is reduced due to closing of a throttle (or accelerator pedal) under the control. Accordingly, the clutch capacity becomes excessive and thus, an engagement shock, a rapid drop of engine rotation speed and the like tend to occur. For dealing with such undesirable phenomenon, there has been proposed a technology (Patent Document 1) in which during the period for engaging the lock-up-clutch, the engaging condition of the lock-up-clutch is controlled while setting, as the engaging capacity of the lock-up-clutch, a value that is provided by subtracting a slippage torque equivalent value of the torque converter from an estimated value of engine output torque inputted to the torque converter.
Although the invention disclosed by Patent Document 1 can cope with a case wherein the engine output is increased or decreased due to an accelerator operation induced by step-returning and increasing actions of the accelerator pedal by a driver, the disclosed invention can't cope with another case wherein the input torque for the torque converter is varied without the accelerator operation by the driver, that is, without change of the output torque itself of the engine.
For example, when an auxiliary device, such as a compressor for an air conditioner or the like that is driven by the engine, is shifted from an operation condition to a stopped condition, a part of the engine output torque that has driven the auxiliary device is added to the torque that is inputted to the torque converter, and thus, the input torque for the torque converter is increased without increase of the engine output torque. In the invention of Patent Document 1, the engaging condition of the lock-up-clutch is controlled based on the torque outputted from the engine, and thus, the technique disclosed by Patent Document 1 can't cope with the increase of the torque inputted to the torque converter. Accordingly, in case of the disclosed technique, undesired pressure shortage phenomenon that would be induced in a transition time for the lock-up condition can't be avoided.
Since rapid engaging of the lock-up-clutch gives the driver a large uncomfortable feeling, it is necessary to assuredly avoid such rapid engaging. Particularly, in case where the engine output torque is lowered upon the step-returning action of the accelerator pedal, the torque inputted to the torque converter is reduced and thus the pressure by which the lock-up-clutch is perfectly engaged is lowered, and thus, the clutch capacity becomes excessive thereby bringing about a rapid perfect engaging of the lock-up-clutch and appearance of the engagement shock and a rapid drop of the engine speed. It is necessary to assuredly eliminate such undesired engagement shock.
In the invention disclosed by Patent Document 1, the fastening condition of the lock-up-clutch is controlled based on an engine output torque that is calculated from a degree of the throttle opening appearing as the result of the step-returning action of the accelerator pedal, and thus, controlling the lock-up-clutch tends to produce a control delay due to a response delay of the hydraulic pressure used for the clutch fastening control. Accordingly, the undesired engagement shock tends to occur before the lock-up-clutch fastening pressure is lowered. It is necessary to assuredly eliminate such defects.